1. Field of the Invention
The present invention relates to an optical device and an image reading apparatus.
2. Related Background Art
A conventional reading apparatus in a facsimile system, scanner system, or any other information processing system often uses an integral image sensor constructed by an illumination unit for illuminating an original, a lens for focusing light reflected by the original and having information of the original, and a sensor for converting the reflected light focused by the lens into an electrical signal.
Such lenses are mainly classified into a reduction-type lens for reducing and forming an image of light reflected by an original, and a full-scale lens for forming a full-scale image of light reflected by an original.
The most popular example of the latter full-scale lens is a so-called lens array in which a large number of lens elements are arrayed to cover a length corresponding to the width of an original.
Strong demand has recently arisen for a compact information processing system such as a facsimile system or scanner system. The reduction-type lens is not desired because it requires a large optical path length, and a lens array, i.e., a full-scale lens having a small optical path length is gaining popularity.
In a lens array, a plurality of cylindrical lens elements having a lens function by setting different refractive indices in the peripheral and central portions are arrayed in line on a flat side plate.
As the methods of manufacturing a gradient index lens element, methods using ion exchange and interdiffusion are available. In the ion exchange method, a glass rod containing exchangeable ions is dipped or brought into contact with a molten salt containing ions contributing to the refractive index distribution to exchange the ions in the glass rod and the molten salt, thereby forming a refractive index distribution in the glass rod. In the interdiffusion method, several types of transparent liquid materials (generally plastics) having different refractive indices are spun by a concentric composite spinning nozzle from the central portion in descending order of refractive index. At the same time, the respective layers are interdiffused to form a refractive index distribution. The resultant lens element is cured with ultraviolet irradiation or the like.
A method of arraying lens elements will be described below. To prevent flare light reflected at the side surface of a gradient index lens element, the outer surface of the lens element is coated with a light absorber having a refractive index nearly equal to that of the lens. The lens elements are then arrayed on a side plate with high precision, and an upper side plate is then placed on the lens elements. The resultant structure is fixed with an adhesive to obtain a lens array.
In arraying the gradient index lens elements, when they are brought into tight contact with each other, a better array can be obtained and lens characteristics such as resolution stabilize without any variation.
The light absorber applied to the outer surface of each lens element have a very small thickness in order to improve the outer dimensional precision of the lens element. When the lens elements are arrayed in tight contact with each other, crosstalk light passes through each element to enter an adjacent lens element to degrade the lens performance.
To prevent this problem, an arbitrary gap is set between two adjacent lens elements, and a light absorption adhesive is filled in the gap to cut the crosstalk light.
The image formation area in which each lens element transmits light reflected by an original onto a sensor IC falls within an image formation radius XO determined by an angular aperture .theta. unique to the lens element. The image formation radii XO of the respective lens elements overlap each other to form a linear image formation area having a width 2 XO in the entire lens array.
The conventional example suffers the following problems.
(1) It is difficult to form an ideal refractive index distribution from the center to the peripheral portion of the gradient index lens element due to manufacturing techniques. The refractive index of the peripheral portion is particularly offset from the ideal value. This becomes one of the causes which decreases the resolution of the lens.
(2) In arraying the lens elements, an arbitrary gap is formed between the two adjacent lens elements. Manufacturing variations which influence the lens characteristics, such as an irregular array pitch of lens arrays and tilt of the lens elements, occur.
(3) In the lens array, as images formed by a large number of lens elements overlap each other to form a final image, the focal depth of the lens array is smaller than that of a single lens element.
(4) The focal depth of the lens array is determined by the angular aperture unique to the lens array. Conventionally, an arbitrary angular aperture cannot be set.